Gas turbines have found a wide use in various applications such as for power generation, for gas compression and many other mechanical drive applications. A gas turbine includes a compressor for compressing ambient air, a combustor burning fuel together with the compressed air and a turbine for driving the compressor. The expanding combustion gases drive the turbine and also result in a net shaft power which may be used for driving a generator, a pump, a compressor, a propeller, or any other device that may be mechanically powered by a rotating shaft.
Gas turbines ingest large quantities of air. With the air follows particles in form of aerosols. Most of the particles exit the gas turbine with the exhaust gases. However, there are particles which may contaminate the compressor gas path of the gas turbine by sticking to the blades and vanes. This contamination also called fouling is most profound in the front end of the gas turbine gas path, i.e. the compressor. The stuck particles will alter the boundary layer air flow over the blades and vanes, thereby changing the aerodynamic properties of the blades and vanes. The changes in aerodynamics result in the gas turbine losing mass flow, thereby reducing the capability of the compressor to compress air, reducing the compressor's efficiency. The compressor of a gas turbine typically consumes 60% of the power available on the shaft. Therefore, a reduction in the compressor efficiency will have a significant impact on the overall performance of the gas turbine. The effects from gas path fouling result in economic losses to the gas turbine operator. It is therefore desired to develop and implement methods and equipment for minimizing fouling.
There are two ways to reduce the effects of fouling. The first is to equip the gas turbine with inlet air filters for reducing contamination that enters the gas path. The second is to wash the particles that are already adhered to the gas path by use of a wash equipment and procedure. In practice, due to the very large quantity of air consumed by a gas turbine, even the best filtering will eventually pass enough contamination for fouling to occur, leading to a need for compressor cleaning.
Washing the gas turbine's gas path on modern gas turbine machines is practiced by injecting a wash liquid upstream of the compressor inlet. By allowing the gas turbine rotor to rotate during wash, the liquid is forced through the compressor and exits at the rear of the gas turbine. The liquid may include water, various chemicals, or a combination thereof. The injection is enhanced by allowing the liquid to be atomized into a fine spray which will distribute the liquid over the entire compressor inlet face. The atomization is provided by nozzles installed permanently on the walls of the air inlet plenum. The liquid is pumped to the nozzles through a pipe or a hose.
Washing is done in two different ways. The most effective way is to wash while the machine is not running at load, but is turning at perhaps 5% of running speed. This mode of washing is called “offline” washing implying that the machine is offline any production. Wash liquid is injected by nozzles directed towards the compressor inlet simultaneously as the machine shaft is slowly being cranked by its starter motor. Fouling is released by the mechanical movements and chemical act of the wash liquid as the liquid slowly moves towards the rear of the machine. This wash method is very effective at restoring the machine performance to prime conditions or near prime conditions. The drawback with the method is if the machine has to be shut down for washing, the cost could be significant for the loss in production revenues.
An alternative wash method is injecting wash liquid as the machine is running. This method is called “online” washing as it implies that the machine is operating in power production mode or in online production. This wash method is not as effective as the offline method for several reasons. First, the online air velocities are very high. A typical air speed at the compressor inlet face is 180 m/s or half the speed of sound. The liquid injection is therefore moved upstream to a position where the air stream is slower and where the liquid is allowed to penetrate into the core of the air stream. Additionally, the turbulence is very strong and liquid is forced towards the walls, where it will not do any good in washing the blades and vanes. Furthermore, the high rotor speed causes liquid impinging on rotor blades to be centrifuged towards the compressor casing where it will not wash the blades. Lastly, the temperature rise within the compressor will soon come to a point where it exceeds the liquid boiling point so that the liquid boils off, disabling any further washing. For a large industrial axial compressor this occurs at about ⅓ of the compressor length. The wash efficacy for online washing is not as good as offline washing due to the difficulties mentioned above and that the wash liquid retention time is very short. Despite the difficulties mentioned, online washing is very popular as it allows washing while the machine production and revenues can be maintained.
The reduced online wash efficacy means that the compressor can be kept clean by daily online washing for a period of time, for example, weeks or months, but build-up of fouling will gradually increase to an unacceptable level. This means that the offline wash capability must also be available to supplement online washing at times when fouling has become significant. Maintaining offline and online wash capability implies one set of nozzles for conducting offline washing and another set of nozzles for conducting online washing. The nozzles will have separate feed lines and valve system making the installation complex and expensive. Further, the maintenance cost will increase.
Additionally, many existing gas turbine installations are currently in place with two nozzle locations to address off line and on line washing needs. These wash applications are typically low pressure (<10 to 15 bar) applications. Such applications have two problems. First, the maintenance of two sets of nozzles is costly, and second, the low pressure application produces a water atomization that is not optimum for cleaning in either the off line or on line conditions.